New cryptanalytic results on the effect of invariance weakness in the KSAm mechanism

In this paper we review the proposed RC4 key scheduling algorithm called KSAm, and we present new cryptanalytic results concerning the effect of the Fluhrer-Mantin-Shamir invariance weakness on KSAm. Taking into account that the KSAm can be integrated within the entire class of known-plaintext attack resistant stream ciphers, whose design is based on the Knuth Fisher-Yates principle for generating a random permutation of a finite sequence in order to obtain a random distribution for the elements of cipher's internal states, we derive probabilistic bounds that formalize this effect on the overall KSAm mechanism.